Ionomers are thermoplastic resins that contain metal ions in addition to organic-chain molecules. Ionomers have solid-state properties characteristic of cross-linked polymers and melt-fabricability properties characteristic of uncrosslinked thermoplastic polymers. Some ionomers are disclosed in U.S. Pat. No. 3,262,272.
Major applications of ionomers are in the areas of packaging and for sporting goods. The use and advantages of such ionomers are described in EP 0 730 622 A1 or WO95/07178 which describes the advantage of ionomers as scuff and scratch resistant top layers.
So far where problems of scratching or scuffing a surface or a film made of an ionomer film or sheet arose, these problems had to be overcome by crosslinking these ionomers by external crosslinking agents such as organic compounds or epoxy and formaldehyde functionalities.
WO 95/11333 discloses a composition made up of ethylene carboxylic acid ionomers and polyfunctional polymerisable olefinic compounds comprising a polyfunctional acrylic or methacrylic ester that provides sufficient crosslinking in order to achieve the scuff resistance necessary to use these ionomers in wear resistant floor tile applications. However, the production of such composition necessitates elevated temperatures and is therefore very difficult to control.
U.S. Pat. No. 3,317,631 describes thermosetting compositions based on ethylene carboxylic acid copolymers and melamine formaldehyde resins giving essentially a thermoset polymer without possibility of thermoplastic processability.
U.S. Pat. No. 3,264,269 teaches a process for crosslinking polymers containing carboxyl groups which comprises imbibing a shaped article of the polymer in a diisocyanate. The disadvantage of this process is its 2 step nature (processing+imbibing) combined with the toxic nature of diisocyanates.
Furthermore, it would be very interesting to have scratch and scuff resistant compositions or films which would not retain dust and dirt during use. It has already been proposed to blend ionomers with antistatic additives. However due to the very polar character of ionomers, a big amount (more than 2 weight percent (wt %) based on total ionomer) of antistatic additives is needed in order to observe any improvement in antistatic performance. Moreover, this process may take several weeks. This solution is therefore not recommended economically speaking.
U.S. Pat. No. 5,037,875 describes an improvement in the antistatic performance of a film of ethylene copolymer ionomers by providing a synergistic additive combination of sorbitan monolaureate and an alkyl phenyl poly(ethylene oxide). However, this combination is for instance not suitable in certain specific applications like floor tiles applications where a surface resistivity of not more than 10E10-10E11 is required.
Therefore, there is still a need for an ethylene copolymer ionomer based composition which would be scratch and scuff resistant and which at the same time would show sufficient antistatic properties in order to be suitable for specific applications requiring very low surface resistivity such as floor tiles applications.
Now, it has been surprisingly found that by combining migrating antistatic agents with an ethylene copolymer ionomer specifically neutralized by Li ions, it was possible to provide a composition having excellent scratch and scuff resistance properties as well as excellent antistatic properties.